Porous solid bodies are becoming increasingly important as adsorbents and catalysts. They differ from other solid bodies in having a hollow structure. This hollow structure is formed by a system of pores. The shape and opening width of these pores ranges from macroscopic recesses and cracks having a diameter of a few .mu.m down to voids having opening widths which are of the order of molecular diameters. The majority of synthetically prepared adsorbents have pores which cover different size ranges and whose size distribution is only rarely homogeneous.
Porous solid bodies are found in numerous chemical families. They include inorganic compounds, such as silicon derivatives, metal oxides, activated carbons and the like, but also porous metals and alloys and partially crosslinked polymers, in particular ion exchange materials.
Porous solid bodies have specific surface areas which are orders of magnitude larger than those of metals. Surface reactions on such solid bodies are consequently accompanied by greater effects. A process with which the chemical nature of the surface or matrix can be changed in a controlled manner by depositing a thin, diffuse coat of metal is therefore of interest.
One of the most important advantages of combining the metal with the hollow geometry of the support material is, for example, that the heat of reaction is simpler to conduct away, by virtue of the high thermal conductivity of the metal, that layer densities and void volumes can be controlled, and that associated pressure losses can be avoided by applying external magnetic fields if used in the form of a fluidized or suspended bed.
The preparation of metallized porous solid bodies is known per se and generally takes the form of loading the porous substrates with ions of transition metals, preferably Ru, Pd, Pt, Ag and Ni, and then treating them with a reducing agent, preferably hydrogen (cf. German Patent Specification No. 1,643,044, German Patent Specification No. 2,553,762, U.S. Pat. No. 3,538,019, U.S. Pat. No. 3,013,987 and East German Patent Specification No. 40,953) or hydrazine, dithionite, boron hydride and the like (cf. German Patent Specification No. 2,849,026, German Patent Specification No. 2,003,522, German Patent Specification No. 18,000,380, French Patent Specification No. 2,270,238, U.S. Pat. No. 4,076,622 Chem. Abstr. 67, 36671 s (1967)) or thermally decomposing the metal compounds applied to the substrates (cf. U.S. Pat. Nos. 3,013,987 and 3,954,883).
These processes, however, have various disadvantages. Their chief defect is that they are not universally applicable. Processes which proceed satisfactorily if, for example, very noble metals, such as Pd and Pt, are used, frequently fail completely when less noble metals, such as Ni and Co, are used.